The present disclosure is directed to a method and apparatus involving a sonde supported transmitter and receiver system with antennas for determining dielectric measurements of formations along a well borehole. After a well has been drilled but before cementing of the well, it is important to make measurements along the borehole to determine properties of the formations penetrated by the well. Important information can be obtained by measuring formation resistivity and the dielectric constant of the materials that make up the formation. One aspect of the present apparatus is a system for measuring dielectric constant. The dielectric of space serves as a reference of 1.00. The dielectric of oil is about 2.00. Various sand and rock formations provide dielectric measurements of about 4.0 to about 10.0. The dielectric of water, however, can range quite high, even as high as 80.0 for relatively pure water. Any impurities in the water may lower the dielectric somewhat, but water can nevertheless be distinguished by the high dielectric.
The present apparatus and method are able to measure dielectric as a means of further distinguishig the nature of the formation encountered along the well borehole. This works even in a mixed region where the formation is made of two materials, one example being a water bearing snad. This process can be used in formations of three materials also such as oil and water in sand. The dielectric is determined by the relative percentage and respective dielectrics making up the formation. In this instance, the value is given by the relationship of D=f.sub.1 d.sup.m +f.sub.2 d.sup.m, where m can be a complex number. In the foregoing, the fractions f.sub.1 . . . f.sub.n represent the respective percentages of the materials in the formation, while the respective material dielectric values are indicated by the symbol d.sub.1 . . . d.sub.n. For instance, if a formation is 1/2 fresh water and the remainder sand, the foregoing equation by itself will not provide fractions of sand and water, but other measurements providing other data enable fractions of sand and water to be mathematically isolated. The present invention is therefore very useful in locating water bearing formations and provides output data indicative of the presence of water, and coupled with other data, can even indicate the percentage fraction of water in a given formation. By estimation of water in a particular formation, other conclusions can be drawn regarding that and adjacent formations which aid and assist in well completion procedures.
The present apparatus is summarized as a microwave transmitting system supported in a sonde having a transmitter antenna which transmits through the adjacent or near formations to a receiver antenna. Microwave frequencies are used and ideal frequencies are in the range of about 30 megahertz to 30 gigahertz or more, the system including a transmitter oscillator connected with an output amplifier providing a continuous wave (CW) signal through a coupling circuit to the transmitter antenna. A portion of the output signal is applied through a mixer to beat with a signal from a local oscillator to provide a reference signal to a measuring circuit. Part of the CW signal applied to the antenna is reflected back into the transmitter circuitry. The formation scatters the transmitted signal dependent on formation electrical parameters and the scattering sends some portion of the CW signal back to the antenna. This contrasts with the absence of scattering where the transmitted signal is directed into space or air. The reflected signal is mixed with the local oscillator signal to produce a reflected signal for the measuring circuit. The received signal is obtained through the receiver antenna and is also mixed with the signal from the local oscillator to provide a signal for the measuring circuit. The measuring circuit makes three separate measurements related to the incident, reflected and transmitted signals all discussed below. These measured signals are converted into a suitable format by an A/D converter and are output for further manipulation to determine values of the dielectric. The foregoing is accomplished at selected radial depths utilizing a sonde supported pad with one or more receiver antennas thereon. Measurements are made along the borehole at a variety of depths. The sonde pad is pushed to the side of the borehole so that it is brought in intimate contact against the sidewall of the well borehole, and such measurements are analyzed as will be described. This provides data regarding the dielectric of formations and is therefore useful in further analysis of the producing formations.